Until recently, a large number of people affected by sensorineural hearing loss of 55 dB or more have been unable to receive adequate therapeutic benefit from any available technology. This problem has been alleviated to some extent by the development of a class of hearing aids generally referred to as implantable hearing instruments, which include, for example, middle ear implants and cochlear implants. Generally, such implantable hearing instruments utilize an implanted transducer to stimulate a component of the patient's auditory system (e.g., tympanic membrane, ossicles and/or cochlea). By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g. U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate causing stimulation of the cochlea through its natural input, the so-called oval window.
Amongst users of implantable hearing instruments, there is a strong desire for a small, fully implantable system. In such hearing instruments, the entirety of the instruments of various hearing augmentation components, including a microphone assembly, is positioned subcutaneously on or within a patient's skull, typically at locations proximate the mastoid process.
As may be appreciated, implantable hearing instruments that make use of an implanted microphone require that the microphone be positioned at a location that facilitates the receipt of acoustic signals. For such purposes, such implantable microphones may be typically positioned in a surgical procedure between a patient's skull and skin, at a location rearward and upward of a patient's ear (e.g., in the mastoid region). Accordingly, the hearing instrument must overcome the difficulty of detecting external sounds (i.e., acoustic vibrations) after attenuation by a layer of skin. In this regard, a subcutaneously located microphone must provide adequate acoustic sensitivity while being covered by a layer of skin between about 3 mm and 12 mm thick.
Further, a subcutaneously located microphone must also be able to discriminate between acoustic sounds and unwanted vibrations. That is, acceleration within patient tissue (e.g., caused by tissue-borne vibration) may cause pressure fluctuations that are commingled with pressure fluctuations caused by acoustic sounds impinging on tissue overlying an implanted microphone. This undesirable commingling of ambient acoustic signals and tissue-borne acceleration signals is at the root of several problems facing the designers of implantable hearing systems.
One particular problem relates to vibrations caused by the implant wearer's voice, chewing or vibration caused by the hearing instrument itself (e.g., an electro-mechanical transducer) that may generate distortion of a wearer's own voice, provide feedback and/or reduce acoustic sensitivity. For example, sound emanating from the vocal chords of a person wearing an implantable hearing instrument passes through the bony structure of the head (i.e., as a vibration) and reaches the implanted microphone of the implantable middle ear hearing system or fully implantable cochlear implant. The vibration reaches the microphone and may induce pressure fluctuations within the skin due to acceleration. Accordingly, such pressure fluctuations may be amplified just as a pressure fluctuation caused by the deflection of the skin's surface by an acoustic sound. In this regard, the implanted microphone detects the combination of these two sources as a single varying pressure. Further, in systems employing a middle ear stimulation transducer, the microphone may produce feedback by picking up and amplifying vibration caused by the stimulation transducer. As such, the bone-borne vibration undesirably limits the maximum achievable gain of the implantable hearing instrument.
In order to achieve a nearly natural quality of the implant wearer's voice and detect acoustic signals with sufficient sensitivity, an implanted microphone needs to isolate acoustic responses from non-acoustic vibration responses. The aim of the present invention is to design an implantable microphone that achieves these goals.